The Motloutse Complex to the southwestern margin of Zimbabwe Craton is a medium-grade terrane in northeastern Botswana. Some studies consider it as part of the Limpopo Complex high-grade terrane between the Kaapvaal and Zimbabwe cratons, while others argue it as an allochthonous terrane separate to the Limpopo Complex. A number of shear zones are postulated to delineate the extents of Motloutse Complex. Except the northern margin (the Shashe shear zone), the rest of the terrane boundaries of the Motloutse Complex are poorly understood. The Magogaphate-Dikalate shear zones are argued to delineate the eastern margin with the Phikwe Complex (sub-terrane of the Limpopo Complex). The western and southern terrane extents of the Motloutse Complex are buried under younger cover rocks. This study characterizes the nature, extent, and timing of shear deformation along the eastern and southern terrane boundaries of the Motloutse Complex. Representative areas from the eastern (Masikati hills), and southern (Radisele-Mogome) regions were selected. A combination of geologic (field, petrographic), geochronologic (40Ar/39Ar amphibole-biotite), and geophysical (aeromagnetic) data were utilized. Neoarchean tonalite gneiss and Mesoarchean mafic (amphibolite-gabbro-leucogabbro anorthosite)-ultramafic (dunite-peridotite-pyroxenite) rocks constitute the dominant rock units in the Masikati hills area in eastern Motloutse Complex. Four concomitant overprint events variably affect the rocks, and include upright isoclinal folding, anatectic, shear deformation and metasomatic components. Shear deformation is prominent along the Abstract Masikati hills. Kinematic indicators, including asymmetric porphyroclasts and S-C fabric, indicate dominant right lateral sense of movement, with minor left lateral movement. The extent of the shear zone (N-S trending Dikalate shear zone), including its branches, is delineated in the aeromagnetic imagery. Application of 40Ar/39Ar geochronology on fabric forming amphibole dates the shear deformation along the eastern terrane boundary of Motloutse Complex at 2013 ± 12 Ma. The NW-SE trending Radisele-Mogome area, beneath the cover rocks, exposes two set of rocks – tonalite gneiss-amphibolite-ultramafic rocks (part of the Motloutse Complex) to the NE, and granodiorite-diorite gneiss-leucogranite (part of the Mahalapye Complex) to the SW. This is well exemplified in the aeromagnetic image, with change in magnetic signature between rocks in the NE and SW parts. Shear deformation and metasomatism variably overprint rocks along the contact zone. Mylonitic deformation is prominent in the SW part. Like the case of the eastern terrane boundary, asymmetric porphyroclasts and S-C fabric indicate a dominant right lateral sense of shear movement. The extent of this shear zone (northwest trending Sunnyside shear zone) and its branch is delineated in the aeromagnetic imagery. 40Ar/39Ar geochronology on biotite from a sheared diorite gneiss from the SW margin of the shear zone indicate an age of 1987 ± 8 Ma. It is argued that both the eastern and southern terrane boundaries of the Motloutse Complex terrane were active in the Paleoproterozoic, with shear deformation slightly earlier along the Dikalate shear zone (~2.013 Ga), relative to the Sunnyside shear zone (~1.987 Ga).

Thesis (MSc of Science in Environmental Science)---Botswana International University of Science and Technology, 2022